What did Salman know of his plans and what might she have done to stop him? Authorities have been trying to pin her down after she apparently gave conflicting statements about what she knew of his intentions in the hours before the attack.
A U.S. attorney plans to bring evidence before a federal grand jury to determine whether charges will be brought, according to two law enforcement officials.
The process could take some time, as investigators need to finish collecting evidence and establish a timeline for Mateen's activities leading to Sunday's attack at the gay club Pulse, in which 49 people were killed, the officials said.
What did the wife know?
Salman, Mateen's second wife, has been cooperating with various law enforcement agencies.
Salman told investigators that Mateen told her he had interest in carrying out a jihadist attack -- but she denied knowing of any specific plans, according to two law enforcement officials.
She initially denied that when Mateen left the house Saturday that she had any idea he was going to do something violent.
But in subsequent statements, Salman conceded she had a suspicion he might be planning an attack, perhaps on Pulse, the officials said. According to one official, she knew "for a while" Mateen had thoughts of wanting to do something violent. He had been talking about it for months, if not years.
According to the second official, Salman told investigators that on Saturday she said she tried to tell him not to commit violence. But she didn't call police.
Mateen and Salman had been married since 2011. They have a 3-year-old son and lived in Fort Pierce, about two hours from where the massacre -- the deadliest shooting in U.S. history -- occurred.
Additionally, it appears that Mateen used Facebook before and during the attack, according to Wisconsin Sen. Ron Johnson.
In a letter to Facebook Chairman and CEO Mark Zuckerberg asking for the company's assistance in the investigation, Johnson said that Mateen used the social networking platform to "search for and post terrorism-related content."
Mateen's messages include declaring his allegiance to ISIS and calls for the U.S. and Russia to stop bombing the terror group, according to the letter.
One message reportedly said "now taste the Islamic state vengeance."
The days and weeks before the crime
Federal investigators are focused on Mateen's and Salman's activities in the days and weeks leading up to rampage.
Investigators believe Mateen made surveillance trips to the club and the Disney Springs shopping complex during Gay Days 2016, a citywide celebration, a law enforcement official said.
Disney security officials told the FBI they believe he also visited Disney World on April 26 to conduct surveillance, the law enforcement official said.
At least one trip to buy ammunition
Salman went with her husband to Pulse and Disney Springs,
where he scouted the places, a law enforcement official said. It's unclear how much she knew about his intentions at that point.
Salman has told investigators she was with her husband on at least one trip to buy ammunition, according to multiple law enforcement officials. When and where that occurred is unclear.
Salman claims she didn't know he was buying ammunition to kill people when she accompanied him, but such purchases were not unusual for him.
CNN has reported that Mateen picked up a Sig Sauer rifle on June 9, four days after he purchased it from the St. Lucie Shooting Center.
Records indicate that Mateen purchased ammunition from that store the same day, according to a law enforcement official. The owner of the shop, Ed Henson, told CNN's Christopher Lett that Mateen visited the store on multiple occasions but he never saw Salman.
He called TV station, a friend and 911
Authorities piecing together the timeline of the killing found that Mateen made several phone calls as his victims lay bleeding to death at the Pulse.
He called a friend to tell him goodbye, two law enforcement officials said. He also called 911 to pledge his allegiance to ISIS.
And he called CNN affiliate News 13
in Orlando to proclaim he was carrying out the massacre on behalf of the terror group.
Producer Matthew Gentili answered the newsroom phone early Sunday.
"I'm the shooter. It's me. I am the shooter," Mateen said, according to Gentili. "I did it for ISIS. I did it for the Islamic State."
Authorities are facing the challenge of trying to glean information from the gunman's cell phone. It was damaged after it was submerged in water and blood, law enforcement officials said.
SWAT officers had severed a water main while tearing down a wall to rescue hostages, officials said.
One official said investigators have now been able to get some data from the shooter's cell phone. Along with data from service providers, they now have a good sense of where he was, what he did and who he communicated with leading up to the attack.
Mateen's contact with gay world
made derogatory comments about gays and expressed outrage over the sight of two men kissing
, according to reports.
But in the months leading up to the carnage, Mateen visited gay chat rooms, messaged people on gay dating apps and even frequented the same gay nightclub he would eventually terrorize, sources said.
Investigators don't know whether the gunman frequented Pulse and chat rooms for personal reasons or for surveillance. Hopper said he had no knowledge of Mateen patronizing any gay nightclubs other than Pulse.
FBI agents are interviewing people who claim they met the gunman on gay dating apps, a law enforcement official said.
Those claims "certainly change the perspective," the source said.
No men have publicly come forward claiming to have had sexual contact with him.
The FBI has given intelligence bulletins to gay clubs in the Orlando area to give them a heightened sense of awareness, Hopper said, and are asking the public for information about the attack and any related threats.
Hopper said there is no evidence to suggest Mateen planned to target any gay club other than Pulse.
Inspired by ISIS or fueled by homophobia?
An analysis of Mateen's electronic devices showed searches for jihadist propaganda, including videos of ISIS beheadings, an official said.
A law enforcement official said the gunman increased his consumption of a variety of jihadist propaganda, such as beheading videos two weeks before the attack.
Before that the source says his consumption was infrequent and he certainly wasn't immersing himself in it. The official described Mateen as an "angry and violent man who in the last few weeks started watching ISIS videos."
A source said he was looking for any excuse to do something violent and he was a "boiling kettle."
But Mateen's father, Seddique Mateen, said his son didn't have ties to the terror group -- which he calls "the enemy of humanity."
He said he didn't believe his son was gay.
"I don't know if he was, if that was his way of his life, but I don't believe so," Seddique Mateen said.
Yet the gunman's first wife, Sitora Yusufiy, said she was not sure about his sexuality.
"It doesn't surprise me that he might be gay. And it doesn't surprise me that he was leading two totally different lives and was in such deep conflict within himself," she told CNN.
"I hope people can truly understand that this is one insane person that did such a tragic thing."